1. Field of the Invention
The invention relates to a process for the continuous conversion of alkynes to mixtures of aromatics, olefins and paraffins useful as fuels or fuel additives. The process utilizes a shape selective zeolite, modified with a metal such as nickel or cobalt, and requires the addition of a hydrogen containing co-reactant in order to achieve continuous single-step conversion of alkynes to higher hydrocarbon product mixtures.
2. Description of Related Art
The continuous catalyzed conversion of acetylene to higher hydrocarbons has been the subject of numerous studies (Tsai, P. and Anderson, 1983; Allenger, Fairbridge et al, 1987; Allenger, McLean et al, 1987; Allenger, Brown et al, 1988). Interest in this process reflects the fact that successful conversion of this type could serve as a possible source of synthetic fuel (Tsai and Anderson, 1983; Allenger, Fairbridge et al, 1987; Allenger, McLean et al, 1987). The synthetic fuel possibility is centered on the fact that acetylene is obtainable in industrial quantities from coal and methane (Tedeschi, 1982). However, as noted explicitly by previous workers, the unavailability of an effective catalyst for continuous acetylene conversion has prevented development of this alternative fuel route (Tsai and Anderson, 1983; Allenger, Fairbridge et al, 1987; Allenger, McLean et al, 1987).
A difficulty encountered with acetylene, or other simple unsaturated hydrocarbon feedstock conversions, is the rapid deactivation of the catalysts employed. This loss of activity is believed to arise primarily from the rapid polymerization of hydrocarbon feedstocks to polycyclic aromatics. These polycyclic aromatics serve as precursors to coke formation and eventual catalytic deactivation. For example, there is an approximate 70% decrease in acetylene conversion using a ZSM-5 catalyst after only 190 min of on-stream conversion at an acetylene space velocity of 460 hr.sup.-1 and reaction temperature of 300.degree. C. (Tsai and Anderson, 1983). Similarly, significant decreases in acetylene conversion over amorphous fluorinated alumina catalysts with time on stream have been reported (Allenger, Brown et al, 1988). In the latter case, catalytic deactivation increased rapidly with increasing acetylene concentration in the reactant stream. Similarly, the ZSM-5 catalyzed conversion of acetylene/hydrogen mixtures revealed a 46% decrease in C.sub.2 H.sub.2 conversion (i.e. from 35.5% to 19%) after only 220 min of on-stream reaction (White et al., 1984).
Zeolite catalyst modification has been explored as a means to alter yield and distribution of aromatized product in feed stream effluents. Nickel-containing zeolites have been tested for ability to convert acetylene to gasoline grade fuel in a single step. Conversion was observed but the yields were low (Seddon, et al, 1986). Zeolite containing framework-incorporated iron is reported to be effective in the conversion of acetylene plus hydrogen mixtures with or without the presence of added water or methanol. However, the conversion effectiveness of this iron containing zeolite is reported to be similar to that of ZSM-5, which exhibits rapid catalytic deactivation. (White et al., 1984) Zeolite catalysts in which cations have been exchanged with Cr(+3) or Cr(+6) ions are known to catalyze the conversion of acetylene to product mixtures containing benzene and alkyl aromatics; however, the catalyst again exhibits rapid deactivation (Chevreau, 1988).
Thus, all previous attempts to achieve continuous catalyzed conversion of acetylene have reported rapid catalytic deactivation. In each case, this loss of catalytic activity is believed to arise from the formation of polycyclic aromatics which, in turn, lead to coke formation on the catalysts. Coke formation in the form of polycyclic aromatics is generally acknowledged to be the prime cause of the surface deactivation of zeolites with consequent decreases in conversion yields.
The desirability of high monocyclic aromatic hydrocarbon content in the acetylene conversion product distribution is based on the octane-enhancing properties of these compounds with respect to the production of synthetic fuels. This property of aromatics is important because of the relatively low octane number of unleaded gasoline due to prohibition of tetraethyl lead addition to increase octane ratings. Therefore efficient processes to provide gasoline fuel mixtures containing aromatics are being sought. Clearly, with respect to the utilization of acetylene for synthetic fuel production, a key aspect of the catalytic conversion upgrade is to generate significant quantities of monocyclic aromatics while simultaneously minimizing formation of the polycyclic aromatics which lead to the catalyst deactivation. This catalyst deactivation has plagued previous attempts at continuous acetylene conversion.